Steel sheets for use in household electrical appliances, construction materials, and automobiles are mainly galvanized steel sheets. The galvanized steel sheets are treated with a liquid mainly composed of chromic acid, dichromic acid, or a salt thereof to prevent white rust and red rust. This chromate treatment provides high corrosion resistance, is inexpensive, and is not complicated in terms of controllability, such as operability. The chromate treatment therefore is widely used. However, because the chromate treatment involves the use of a regulated substance, hexavalent chromium, the application of the chromate treatment has gradually been restricted.
In this situation, various techniques have been proposed to prevent white rust in a galvanized steel sheet without using hexavalent chromium, as described, for example, in Japanese Examined Patent Application Publication No. 3405260 and Japanese Unexamined Patent Application Publication Nos. 2001-181860, 2003-13252, and 2003-105562.
In these techniques, a metallic compound for preventing rust or a resin that can form a dense film retarding the permeation of corrosive factors, such as oxygen, water, and salts are selected.
When surface-treated, galvanized steel sheets are used in the applications described above, alkaline detergents are used to remove lubricating oil used during processing and deposited foreign matter. In particular, strong alkaline degreasing solutions are being increasingly used to remove lubricating oil in a short time and thereby increase production efficiency. Furthermore, organic solvents, such as alcohols and ketones, are sometimes used to remove deposited foreign matter after processing or to erase oil-based or water-based markings on processed products. Furthermore, when a consumer uses a household electrical appliance, an oil stain or dust mars the appearance over time. Thus, a cleaning liquid containing an organic solvent or an alkaline cleaning liquid containing a detergent is sometimes used to remove the oil stain or the dust.
In this situation, a surface-treated film of a plated steel sheet must have resistance to an alkaline solution or an organic solvent (alkali resistance or solvent resistance). Low resistance may result in dissolution or abrasion of the surface-treated film. Such damage to the surface-treated film causes a decrease in corrosion resistance during long-term use, thus resulting in inferior quality. Furthermore, the elution of a component from the surface-treated film to the organic solvent causes whitening and unevenness, thus resulting in a nonuniform appearance and poor design.
However, in known techniques, including those described in the four patent documents, in which no chromate is used, a surface-treated galvanized steel sheet cannot simultaneously have corrosion resistance, alkali resistance, and solvent resistance, as well as long-term high corrosion resistance. Hence, there is a strong demand for a surface-treated galvanized steel sheet that is treated without using chromate and is excellent in terms of corrosion resistance, alkali resistance, and solvent resistance.
Accordingly, it is an object of the present invention to solve the problems of the related art and provide a surface-treated galvanized steel sheet that is free from hexavalent chromium and is excellent in terms of corrosion resistance, alkali resistance, and solvent resistance.